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><H1
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><A
NAME="RUNTIME-CONFIG-COMPATIBLE"
>18.13. Version and Platform Compatibility</A
></H1
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-COMPATIBLE-VERSION"
>18.13.1. Previous PostgreSQL Versions</A
></H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-ARRAY-NULLS"
></A
><TT
CLASS="VARNAME"
>array_nulls</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
>        This controls whether the array input parser recognizes
        unquoted <TT
CLASS="LITERAL"
>NULL</TT
> as specifying a null array element.
        By default, this is <TT
CLASS="LITERAL"
>on</TT
>, allowing array values containing
        null values to be entered.  However, <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> versions
        before 8.2 did not support null values in arrays, and therefore would
        treat <TT
CLASS="LITERAL"
>NULL</TT
> as specifying a normal array element with
        the string value <SPAN
CLASS="QUOTE"
>"NULL"</SPAN
>.  For backward compatibility with
        applications that require the old behavior, this variable can be
        turned <TT
CLASS="LITERAL"
>off</TT
>.
       </P
><P
>        Note that it is possible to create array values containing null values
        even when this variable is <TT
CLASS="LITERAL"
>off</TT
>.
       </P
></DD
><DT
><A
NAME="GUC-BACKSLASH-QUOTE"
></A
><TT
CLASS="VARNAME"
>backslash_quote</TT
> (<TT
CLASS="TYPE"
>enum</TT
>)</DT
><DD
><P
>        This controls whether a quote mark can be represented by
        <TT
CLASS="LITERAL"
>\'</TT
> in a string literal.  The preferred, SQL-standard way
        to represent a quote mark is by doubling it (<TT
CLASS="LITERAL"
>''</TT
>) but
        <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> has historically also accepted
        <TT
CLASS="LITERAL"
>\'</TT
>. However, use of <TT
CLASS="LITERAL"
>\'</TT
> creates security risks
        because in some client character set encodings, there are multibyte
        characters in which the last byte is numerically equivalent to ASCII
        <TT
CLASS="LITERAL"
>\</TT
>.  If client-side code does escaping incorrectly then a
        SQL-injection attack is possible.  This risk can be prevented by
        making the server reject queries in which a quote mark appears to be
        escaped by a backslash.
        The allowed values of <TT
CLASS="VARNAME"
>backslash_quote</TT
> are
        <TT
CLASS="LITERAL"
>on</TT
> (allow <TT
CLASS="LITERAL"
>\'</TT
> always),
        <TT
CLASS="LITERAL"
>off</TT
> (reject always), and
        <TT
CLASS="LITERAL"
>safe_encoding</TT
> (allow only if client encoding does not
        allow ASCII <TT
CLASS="LITERAL"
>\</TT
> within a multibyte character).
        <TT
CLASS="LITERAL"
>safe_encoding</TT
> is the default setting.
       </P
><P
>        Note that in a standard-conforming string literal, <TT
CLASS="LITERAL"
>\</TT
> just
        means <TT
CLASS="LITERAL"
>\</TT
> anyway.  This parameter only affects the handling of
        non-standard-conforming literals, including
        escape string syntax (<TT
CLASS="LITERAL"
>E'...'</TT
>).
       </P
></DD
><DT
><A
NAME="GUC-DEFAULT-WITH-OIDS"
></A
><TT
CLASS="VARNAME"
>default_with_oids</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
>        This controls whether <TT
CLASS="COMMAND"
>CREATE TABLE</TT
> and
        <TT
CLASS="COMMAND"
>CREATE TABLE AS</TT
> include an OID column in
        newly-created tables, if neither <TT
CLASS="LITERAL"
>WITH OIDS</TT
>
        nor <TT
CLASS="LITERAL"
>WITHOUT OIDS</TT
> is specified. It also
        determines whether OIDs will be included in tables created by
        <TT
CLASS="COMMAND"
>SELECT INTO</TT
>. The parameter is <TT
CLASS="LITERAL"
>off</TT
>
        by default; in <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 8.0 and earlier, it
        was on by default.
       </P
><P
>        The use of OIDs in user tables is considered deprecated, so
        most installations should leave this variable disabled.
        Applications that require OIDs for a particular table should
        specify <TT
CLASS="LITERAL"
>WITH OIDS</TT
> when creating the
        table. This variable can be enabled for compatibility with old
        applications that do not follow this behavior.
       </P
></DD
><DT
><A
NAME="GUC-ESCAPE-STRING-WARNING"
></A
><TT
CLASS="VARNAME"
>escape_string_warning</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
>        When on, a warning is issued if a backslash (<TT
CLASS="LITERAL"
>\</TT
>)
        appears in an ordinary string literal (<TT
CLASS="LITERAL"
>'...'</TT
>
        syntax) and <TT
CLASS="VARNAME"
>standard_conforming_strings</TT
> is off.
        The default is <TT
CLASS="LITERAL"
>on</TT
>.
       </P
><P
>        Applications that wish to use backslash as escape should be
        modified to use escape string syntax (<TT
CLASS="LITERAL"
>E'...'</TT
>),
        because the default behavior of ordinary strings is now to treat
        backslash as an ordinary character, per SQL standard.  This variable
        can be enabled to help locate code that needs to be changed.
       </P
></DD
><DT
><A
NAME="GUC-LO-COMPAT-PRIVILEGES"
></A
><TT
CLASS="VARNAME"
>lo_compat_privileges</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
>        In <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> releases prior to 9.0, large objects
        did not have access privileges and were, in effect, readable and
        writable by all users.  Setting this variable to <TT
CLASS="LITERAL"
>on</TT
>
        disables the new privilege checks, for compatibility with prior
        releases.  The default is <TT
CLASS="LITERAL"
>off</TT
>.
       </P
><P
>        Setting this variable does not disable all security checks related to
        large objects &mdash; only those for which the default behavior has
        changed in <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 9.0.
        For example, <TT
CLASS="LITERAL"
>lo_import()</TT
> and
        <TT
CLASS="LITERAL"
>lo_export()</TT
> need superuser privileges independent
        of this setting.
       </P
></DD
><DT
><A
NAME="GUC-QUOTE-ALL-IDENTIFIERS"
></A
><TT
CLASS="VARNAME"
>quote_all_identifiers</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
>        When the database generates SQL, force all identifiers to be quoted,
        even if they are not (currently) keywords.  This will affect the
        output of <TT
CLASS="COMMAND"
>EXPLAIN</TT
> as well as the results of functions
        like <CODE
CLASS="FUNCTION"
>pg_get_viewdef</CODE
>.  See also the
        <TT
CLASS="OPTION"
>--quote-all-identifiers</TT
> option of
        <A
HREF="app-pgdump.html"
>pg_dump</A
> and <A
HREF="app-pg-dumpall.html"
><SPAN
CLASS="APPLICATION"
>pg_dumpall</SPAN
></A
>.
       </P
></DD
><DT
><A
NAME="GUC-SQL-INHERITANCE"
></A
><TT
CLASS="VARNAME"
>sql_inheritance</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
>        This controls the inheritance semantics.  If turned <TT
CLASS="LITERAL"
>off</TT
>,
        subtables are not accessed by various commands by default; basically
        an implied <TT
CLASS="LITERAL"
>ONLY</TT
> key word.  This was added for
        compatibility with releases prior to 7.1.  See
        <A
HREF="ddl-inherit.html"
>Section 5.8</A
> for more information.
       </P
></DD
><DT
><A
NAME="GUC-STANDARD-CONFORMING-STRINGS"
></A
><TT
CLASS="VARNAME"
>standard_conforming_strings</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
>        This controls whether ordinary string literals
        (<TT
CLASS="LITERAL"
>'...'</TT
>) treat backslashes literally, as specified in
        the SQL standard.
        Beginning in <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 9.1, the default is
        <TT
CLASS="LITERAL"
>on</TT
> (prior releases defaulted to <TT
CLASS="LITERAL"
>off</TT
>).
        Applications can check this
        parameter to determine how string literals will be processed.
        The presence of this parameter can also be taken as an indication
        that the escape string syntax (<TT
CLASS="LITERAL"
>E'...'</TT
>) is supported.
        Escape string syntax (<A
HREF="sql-syntax-lexical.html#SQL-SYNTAX-STRINGS-ESCAPE"
>Section 4.1.2.2</A
>)
        should be used if an application desires
        backslashes to be treated as escape characters.
       </P
></DD
><DT
><A
NAME="GUC-SYNCHRONIZE-SEQSCANS"
></A
><TT
CLASS="VARNAME"
>synchronize_seqscans</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
>        This allows sequential scans of large tables to synchronize with each
        other, so that concurrent scans read the same block at about the
        same time and hence share the I/O workload.  When this is enabled,
        a scan might start in the middle of the table and then <SPAN
CLASS="QUOTE"
>"wrap
        around"</SPAN
> the end to cover all rows, so as to synchronize with the
        activity of scans already in progress.  This can result in
        unpredictable changes in the row ordering returned by queries that
        have no <TT
CLASS="LITERAL"
>ORDER BY</TT
> clause.  Setting this parameter to
        <TT
CLASS="LITERAL"
>off</TT
> ensures the pre-8.3 behavior in which a sequential
        scan always starts from the beginning of the table.  The default
        is <TT
CLASS="LITERAL"
>on</TT
>.
       </P
></DD
></DL
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="RUNTIME-CONFIG-COMPATIBLE-CLIENTS"
>18.13.2. Platform and Client Compatibility</A
></H2
><P
></P
><DIV
CLASS="VARIABLELIST"
><DL
><DT
><A
NAME="GUC-TRANSFORM-NULL-EQUALS"
></A
><TT
CLASS="VARNAME"
>transform_null_equals</TT
> (<TT
CLASS="TYPE"
>boolean</TT
>)</DT
><DD
><P
>        When on, expressions of the form <TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>expr</I
></TT
> =
        NULL</TT
> (or <TT
CLASS="LITERAL"
>NULL =
        <TT
CLASS="REPLACEABLE"
><I
>expr</I
></TT
></TT
>) are treated as
        <TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>expr</I
></TT
> IS NULL</TT
>, that is, they
        return true if <TT
CLASS="REPLACEABLE"
><I
>expr</I
></TT
> evaluates to the null value,
        and false otherwise. The correct SQL-spec-compliant behavior of
        <TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>expr</I
></TT
> = NULL</TT
> is to always
        return null (unknown). Therefore this parameter defaults to
        <TT
CLASS="LITERAL"
>off</TT
>.
       </P
><P
>        However, filtered forms in <SPAN
CLASS="PRODUCTNAME"
>Microsoft
        Access</SPAN
> generate queries that appear to use
        <TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>expr</I
></TT
> = NULL</TT
> to test for
        null values, so if you use that interface to access the database you
        might want to turn this option on.  Since expressions of the
        form <TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>expr</I
></TT
> = NULL</TT
> always
        return the null value (using the SQL standard interpretation), they are not
        very useful and do not appear often in normal applications so
        this option does little harm in practice.  But new users are
        frequently confused about the semantics of expressions
        involving null values, so this option is off by default.
       </P
><P
>        Note that this option only affects the exact form <TT
CLASS="LITERAL"
>= NULL</TT
>,
        not other comparison operators or other expressions
        that are computationally equivalent to some expression
        involving the equals operator (such as <TT
CLASS="LITERAL"
>IN</TT
>).
        Thus, this option is not a general fix for bad programming.
       </P
><P
>        Refer to <A
HREF="functions-comparison.html"
>Section 9.2</A
> for related information.
       </P
></DD
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